Starship Asterisk*

Yes, but can a stable gas giant planet exist in the "habitable zone"? I recognize that a few of the earliest identified planets around other stars appear to be extra large gas giants, ID'ed because of their close in, rapid orbits, which caused the stars to "wobble" measurably.

However, over the long run, the gravity has to be pretty extreme to not have a problem of hydrogen escaping from the upper atmosphere. This is believed to be the primary reason that the outer planets are gas giants, while the inner ones are rocky. All the hydrogen "cooked off", because in a normal (Boltzmann?) distribution, a substantial minority of the hydrogen in the upper atmosphere achieves escape velocity through solar heating.

This is the same reason that Mars (with its low gravity) is unable to even retain its carbon dioxide, and the Moon has no atmosphere at all, while Earth and Venus have relatively dense atmospheres, although very poor in gases with molecular weights below ionized nitrogen. Actually, hot Venus has not even retained much nitrogen or oxygen.

Also, does any large gas giant moon have substantial satellites of its own. I think not. For large, stable co-orbiting satellites, the orbit around the primary would have to be very long.

Good point randall, but with a "if" in it ,like, if an earth size planet with a moon to stabilise its rotation (moon being in the other side of the planet for now ) and all that in orbit around a jupiter size..... etc... May be then it can work? The sea might be calm and stable at each high tide and low tide. If not then it was a good dream.

I always thought that it was a photo, taken through a telephoto lens...

Lenses tend to enlarge everything equally, moon, sun, background etc.

It might have existed some 4.3 billion years ago after the moon had just formed and was (17 times?) closer to us than it is now.

Good thought, but both earth and moon would have been rather hotter and more shapeless (plastic crust?); not likely to have calm, liquid standing water or even a transparent atmosphere. And where is the earthshine?

It might also be an earth size planet orbiting a giant jupiter wich orbit itself in the living zone of that star.

If that were the case, tidal action would make the moon we are on a rough place, probably with orbitally coupled rotation, meaning very long days and nights with attendant weather extremes, unless atmospheric pressure and density were very high with lots of cloud cover, like Venus. Weather would also get interesting anytime the orbit passed in and out of the planet's shadow for a day or so.

Probably not a place you would want to live.

See Hideway may not be so far away for more discussion of the same topic.

Dang!

Depending upon the angular size of the photo, either that sun is really far away, in which case everything should be frozen, and/or that moon is either really huge or really close, in which case you could expect massive tides on both bodies, enough to cause some nasty wave action on the primary, possibly Io-like volcanic activity on the secondary, and probably orbital coupling of rotation of even the larger body, meaning a fast lunar orbit and long days.

Not a place I would want to live. Many of these sci-fi images fail to get cosmic scales right.

More like a double planet, than earth with a small satellite. Incidentally, ours is often considered a double planet - it has a vastly higher ratio of satellite to primary mass than any other large object in the solar system. Pluto - Charon and some other big Kuiper belt objects fall into the same category.


Also, in addition to being featureless, the dark side of the very close moon should be illuminated by a lot of "earthshine" reflected from the full earth. The dark side of new moons is readily visible to us. Unless the planet has extraordinary low albedo.

You are fully right. So many glitches. To nice. To artificial. But the conjunction could be possible. What POD officially stated is in detail not fully correct.

It might also be an earth size planet orbiting a giant jupiter wich orbit itself in the living zone of that star.

A beautiful rendition to be sure...

Nice idea about post-sunset and Venus! A few extra nit-picky science flaws: 1) an airless Moon will not show a 180 degree arc at small elongations from the illuminating body. The crater rims on/near the limb cast shadows reducing the surface brightness (or eliminating it!) of the arcs. And there are no craters on this Moon 2) there is little to no scattering in the atmosphere. Both the objects should be yellow (scattering of the blue light). Images that have a white Moon close to the horizon are a dead giveaway for being fake. 3) The reflection of the yellow object in the water is brighter than the original
4) the foreground is illuminated but the scene is backlit

All that aside, I often have dreams with these sorts of flaws but I never seem to catch on. <grin>

Of course it is not on the North Pole. It must be on the equator. The smal star is of course not the sun but could be Venus? All nice aligned with a sun just set.

Welcome too.

It might have existed some 4.3 billion years ago after the moon had just formed and was (17 times?) closer to us than it is now.

Welcome to APOD Rich! Yes it is a beautiful Picture.
http://antwrp.gsfc.nasa.gov/apod/ap060620.html
Orin

Hi,

If I remember well, the picture was made using Terragen, a powerful landscape generator. I've been using this software for more than five years, so, when I saw the APOD, I wanted to see the discussion around the pic

The APOD for June 20, 2006 was Hideaway. In the description it said that the picture was imagineary (I didn't know that) and could not have been taken anywhere on earth because the moon and sun were the same angular size. I always thought that it was a photo, taken through a telephoto lens, which (in my mind) explained why the moon was so large.

In any case, it's a beautiful picture. I just joined today but have been looking at and learning from APOD for years now. Its a great site and I appreciate all the efforts of everyone who contributes to it. Thanks.

Rich